A STRUCTURAL INTEGRITY EVALUATION OF THE TANK FARM WASTE TRANSFER SYSTEM Page: 66 of 86
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WSRC-TR-2005-00532 March, 2006
Page 59 of 69
measured over a given surface area such that depths and nearest neighbor pits could be
more accurately determined.
Step 1. Determine the pitting factor (PF).
The PF is determined from the following formula:
PF = Maximum Pit Depth/Depth of General Corrosion (4)
The minimum wall thickness for the jacket was 0.127". Given the average wall thickness
of 0.24", the Maximum Pit Depth was 0.113". Assuming that the jacket was originally
the nominal thickness of 0.25", the Depth of General corrosion was 0.01" (Note: several
of the wall thickness measurements were well above nominal). Thus, the PF was 11.3.
Step 2. Determine the constant K for the pitting equation.
Pit depth has been shown to increase with time in a parabolic fashion [111] according to
the following equation:
Maximum Pit Depth = K rs (5)
where is is the time in service. The jacket had been in service for approximately 20
years. Thus,
K = (0.113")/(20 years) = 0.0253"/yr
Step 3. Determine the total penetration as a function of time, rs.
Total penetration (tp) through the jacket wall is determined by the following equation:
tp=[(PF+ 1)K s ]/PF (6)
tp = [12.3 * 0.0253"/yr is ]/ 11.3 = 0.0275"/yr is
Step 4. Determine the time to penetrate through-wall.
The equation from Step 3 can be rearranged such that is becomes the time to penetrate
through the wall.
is = (tp/0.0275"/yr ) 2 (7)
Assuming that the jacket was initially the nominal thickness of 0.25", the time to
penetrate through-wall is approximately 80 years.
For comparison purposes, the time to failure for a jacket with a thinner wall was
determined (e.g., Line #106). Substituting the nominal wall thickness of 0.156" into
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Wiersma, B. A STRUCTURAL INTEGRITY EVALUATION OF THE TANK FARM WASTE TRANSFER SYSTEM, report, March 9, 2006; United States. (https://digital.library.unt.edu/ark:/67531/metadc836317/m1/66/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.